Why Is Delta-Wye Transformer Used In Distribution Transformer

Dear Mr. RAMConsult,

During our college study days, my Professors have taught that STAR- CONNECTION - because of 120 Degree, Phase Angle the 3rd Harmonics are totally nullified. But your point is Delta connection nullifies 3rd Harmonics. Pl. clarify.

DHAYANANDHAN.S

Mr. Dhayanadhan:

In a delta the third harmonic currents circulate within the delta and turn into heat thereby increasing the loading on the cooling system.

Wye connections are different because of the neutral point (grounded/ungrounded), where it appears (Primary/Secondary), and whether the source is grounded or not, making a total of sixteen different scenarios (which I will not bore you with, for more info see the Westinghouse T&D Handbook).

To simplify(?), for certain wye-wye grounding configurations, the third harmonic current has no choice but to pass through the transformer, appearing in the phase to neutral voltage but not in the phase to phase voltage (which is probably the case that your Professors taught you). If there is a delta tertiary then the third harmonics will be suppressed regardless of either wye windings' grounding.

In general from ibid "..The paths permitting the flow of third harmonic currents are determined by the system and transformer zero-sequence circuits..." (which are even more arcane (mysterious, secretive) than the above discussion!)

Dear Mr. RAMConsult,

Thank you very much for your reply. Indeed a good amount of explanation given by you, which will help to up-date the knowledge. What we studied in the college and what we see in the Industry varies some times.

You have mentioned "...........making a total of sixteen different scenarios (which I will not bore you with, for more info see the Westinghouse T&D Handbook)."

I never feel bored, and I want to know more about the 6 different Scenario. I also request you to give me the web-link where I can down-load the Westinghouse T&D Hand Book or you can send to my cr4 personal messaging system.

Thanks,

DHAYANANDHAN.S

Note: Sorry for delay in acknowledging your reply.

GA

Perfect answer.

The various types of transformers are used for many different reasons. Some economic, some may be phase sequencing, along with the many other requirements a transformer is used for.

Unfortunately for you, you have to research why they are used in each circumstance.

Tell me why you think Yy is used on MV instrumentation or why ∆Yn is used for MV/LV distribution? I'll help if you help yourself.

Wye xformers are used in 4wire systems for increased current capacities as their leg to leg potential value is derated by 1.713. As the neutral is center tapped the unbalanced loads are a non issue. Delta xformers are better for transmission as their leg to leg potentials are higher to compensate for I2R losses. Ie 120/208 in commercial and 120/240 in residential.

The type of transformer connections (wye or delta) has each own individual and unique characteristics.. They are connected / terminated depending on the needs or as governed by its intended application. Intended application therefore will determine how, to take advantage of this inherent and fundamental connection characteristics..

You need to study to better understand the basic theories relating to transformers.